Mitomycins are known as antibiotics having, in general, antibacterial and antitumor activities. Among known mitomycin derivatives, are those where one hydrogen in the methyl group at the 6-position is substituted by alkoxy or alkylthio, which derivatives are illustrated in JP-A-167282/90, and represented by the following formula: ##STR2## in which W' represents alkoxy or alkylthio.
The term "JP-A" as referred to herein means "Japanese Published Unexamined Patent Application".